There are various building regulations and personnel safety requirements concerning sound absorption to consider for different types of premises. Various types of panels have been made with the intention of giving a large amount of sound absorption. For instance, there are perforated panels which are used together with a back layer of mineral wool. Those combinations give an excellent sound adsorption, but in most cases only within a small limited range of frequencies. In special premises like auditoriums, theaters, etc. different areas are provided with absorbents having different active frequency ranges for fine adjustment of reverberation time. Such a partial area may have a large absorption within a small range of frequencies, or a special frequency absorption diagram.
In most premises such as large workshops, assembly halls and lobbies, etc. the entire ceiling area is normally panelled with only one type of absorbent. It is better if this has moderately good absorption in a large range of freqencies, rather than a very good absorption within a small range of frequencies.
A common type of absorbent is a lacunaria hanging in special carrier systems or pendulums and forming a surface for a thick layer of loose mineral wool.
When these kinds of ceilings are assembled, it is very important that the mineral wool is correctly positioned, in order to reach the intended absorption effect. Electrical lines and ventilation channels are often passed within the space between the ceiling and the lacunaria. During maintenance jobs on these systems, parts of the lacunaria has to be taken down and there is always a risk that the mineral wool is not properly returned to its position by an electricity or ventilation technician. Perforated panels are also known, wherein the back of the panel has a loosely laying or partially bonded sheet of cloth, instead of the mineral wood. This measurement results in a simplified assembly and adjustment, but not in a more efficient absorption.
It is more difficult to get a broad band absorbent with perforated panels of plasterboard and other building boards, than with the alternative, more expensive panels of metal and plastics. The reason for this is that the latter can be made with much less thickness. These thin panels will, however, after repainting have a considerably reduced ability to absorb sound, since paint can adhere to the cloth.
There is a clear need for widening the range of sound absorbing frequencies considerably for perforated plasterboard.
Three parameters have to be considered when a good sound absorption in a wide range of frequencies is desired.
1. The extent of perforation, which has to be as large as possible.
2. The thickness of the panel, which has to be as small as possible.
3. The distance (gap) between the primary ceiling and the lacunaria, which has to be large.
What are the implications for these three parameters?
1. Thin panels of metal or plastics may be perforated to a degree of about 30%. With plasterboard, in order to maintain strength, a perforation degree of more than 20% is not possible.
2. Panels of metal or plastics are available in thicknesses of less than 2 mm, whereas the plasterboards have a thickness of between 6 and 18 mm.
3. With thin panels having a large degree of perforation, good absorption may be obtained with gaps from 30 mm and upwards. To obtain similar data for plasterboard, a minimal gap of 70 mm is necessary. In both cases the absorption range of frequencies is improved considerably when the gap is increased to about 200 mm. Thereafter the improvements are minimal.
When all parameters 1-3 are favorable, broad band absorbents may be obtained with mineral wool and loosely fastened, thin cloth on the panel.
For plasterboard, parameters 1 and 2 have always been unfavorable, and so far, no method is known to obtain good absorption in a board frequency range.
The Swedish patent applications SE No. 402 142, SE No. 404 540, SE No. 407 957 and SE No. 440 524 and West German application DE No. 3 242 940 relates to resonance absorbents having a thin fibre cloth instead of a layer of mineral wool, but in all of these prior art documents it is emphasized that the fibre cloth should be fixed to a very thin perforated panel, e.g. of a thin metal plate. In those cases a desirable specific flow resistance is given for the perforated panel with cloth, it is about 500 Nsm.sup.-3. These absorbents are therefore best suited when a large absorption near the resonance frequency is desired, rather than a broad band absorption.
For the sound absorbing baffle wall according to SE No. 404 540, a value of 400 Nsm.sup.-3 is given as suitable for obtaining a maximum absorption in the frequency range of 500-1000 Hz where the intensity of the traffic noise is largest.